1. Field of the Invention
The present invention relates generally to the field of ballast circuits for groups of ultraviolet lamps. The invention specified relates particularly to applications using four or more ultraviolet lamps and several unique application features it can provide in ultraviolet transilluminators and ultraviolet crosslinkers.
2. Description of Prior Art
Transilluminators and ultraviolet crosslinkers utilize ultraviolet lamps in their measurements of electrophoresis. Electrophoresis is an analytical tool that is used in the study of bacteria, viruses, and in protein differentiation, purification and nucleic acid studies. Electrophoresis involves the separation of charged molecules under the influence of an applied electric field.
Visualization of electrophoresis with a transilluminator occurs by the optical absorption method or by the fluorescence method using an ultraviolet transilluminator.
Alternatively, electrophoresis is also conducted with a crosslinker as follows: Nucleic acid transferred to a membrane is exposed to ultraviolet light which causes formation of a stable bond between bound molecules and the nylon membrane.
All of the present commercial transilluminators and crosslinkers use fluorescent ultraviolet lamps that utilize electromagnetic ballasts. With these electromagnetic ballasts the tubes will flicker momentarily while they light. This flicker period will vary as some function of how long the unit has been on and the flicker period will vary with aging of the ultraviolet lamps. Such flicker is undesirable in the transilluminator when visualization of a gel requires differentiation of shadings. The flicker is undesirable in the crosslinker where the calibration is determined by measuring the output intensity over a time period. Such flicker is undesirable in devices that must put out a measured repeatable pulse of ultraviolet illumination.
Ultraviolet lamps used in these devices are basically fluorescent lamps that have a negative internal resistance characteristic once the gas in the lamp is ionized. This means that as current increases through the lamp, the resistance of the lamp decreases. This resistance decrease causes the current to further increase so that, unless some current-limiting ballast is provided, the lamp will be destroyed by excess current.
Thus, a ballast system is required which will enable the lamp to operate at a sufficiently high current for proper illumination, but will prevent the current from increasing to a level at which the lamp will destroy itself. In addition, a fluorescent lamp exhibits a very high effective internal resistance until the gas within the lamp ionizes, at which time a much lower resistance is presented. For that reason, the fluorescent lamp requires a high starting voltage in order that the lamp may be ignited.
For many years, iron-core transformer ballast systems have been utilized to control fluorescent lamp current. Such designs were the only economical type available which were capable of providing a high starting voltage and at the same time, capable of limiting the operating current to an appropriate level. These iron-core ballast circuits were used extensively despite undesirable characteristics including low power efficiency, an audible buzz and high weight.
There have been a number of approaches to improve the efficiency of fluorescent lamp ballast systems. The newest approach has led to the development of solid state high-frequency electronic ballast systems. High frequency is advantageous because the ballast system and the fluorescent lamps are more efficient at higher frequencies. Solid state high frequency ballasts have become available to operate ordinary fluorescent lamp fixtures. These recent solid state ballast systems have the advantage over the prior art iron-core ballast with smaller size, lower weight, no audible noise and increased power efficiency. The disadvantage of the solid state ballast is higher cost compared to iron-core ballasts.
This cost difference however is more than compensated for in industrial and commercial lighting systems by the saving in operating electrical energy cost. The existing electronic ballasts are designed for commercial and industrial applications and are not practical for small electronic instrument applications where low cost and small size are major concerns and reduction of power consumption is not important.
The inventors have designed an inexpensive electronic ballast system which is small, flickerless and provides additional features not present in the ballasts used in the electrophoresis measurement industry at the present time. The present invention reduces ballast cost by enabling one ballast control to drive four or more ultraviolet lamps using capacitors on the output lines to control the individual lamp current. Through the use of these capacitors, many other unique features become available and the circuit design is simplified because there is no concern over power factor due to the low power consumption of lamps used in measuring instruments.
3. Description of a Ballast for Fluroescent Lamps
A ballast is a current and voltage regulating device that is used with a fluorescent lamp to perform these main functions:
1. It transforms line voltage to the proper open circuit voltage necessary for a particular lamp that it will operate. PA1 2. It provides a specific amount of electrical energy to preheat the lamp electrodes. PA1 3. It supplies a controlled high voltage to initiate the lamp arc. PA1 4. It controls lamp current and operating voltage within the limits prescribed by the lamp manufacturer. PA1 1. The electronic ballast is complete on a single small circuit board. PA1 2. The electronic ballast can drive as many as 6 tubes at a time. PA1 3. The electronic ballast can drive different wattage tubes with a minor component change. PA1 4. The electronic ballast operates at a frequency greater than 25 kHz, thereby eliminating any flicker during start or operation. PA1 5. The circuit allows for all varying inputs by simply removing or installing one jumper cable. The same basic circuit is applicable to drive 4, 5, or 6 lamps at one time. Each lamp has its current controlled by an individual circuit element. PA1 6. The electronic ballast will provide "instant on" with no startup flicker. PA1 7. There will be no audible ballast hum. PA1 8. The circuit design includes electromagnetic interference filtration, surge protection, and inrush current limiting. PA1 1. U.S. Pat. No. 4,370,600 by Zansky describes a two wire electronic circuit for controlled dimming of the lamps from zero to maximum. This design utilizes additional windings of the transformer to control lamp current. It discloses circuit diagrams for a high frequency solid state dimmable fluorescent ballast which utilizes a resonant bridge invertor to provide high frequency sinusoidal power to the lamps. He includes a current limiting resistor in his disclosure. PA1 2. U.S. Pat. No. 4,394,603 by Widmayer discloses an energy conserving system designed primarily for power savings. The concept and design is to have the intensity adjusted based on ambient light. The basic application is for overhead lighting and conservation of power. He discloses a starting circuit wherein a lamp and a resistor act as the ballast that limits the current. In some cases he describes a transistor ballast and control circuit to control current. PA1 3. U.S. Pat. No. 4,525,648 by De Biji discloses a DC/AC converter using transistors and inductors along with zener diodes as frequency converters and also using timing circuits. This properly heats the lamp electrodes before the lamp ignites. It is not clear that the circuits act as ballasts. PA1 4. U.S. Pat. No. 4,847,535 by Wisbey discloses a hybrid ballast to prevent electrical shock with lamps connected in series. The design basically limits the voltage at the connection to the lamp when one of the lamps in series is disconnected. PA1 5. U.S. Pat. No. 4,937,502 by Pro describes an electronic ballast with a power circuit having magnetic transformers, parallel lamps and a FET circuit for fluorescent lights utilized in aircraft applications. Power factor is critical and controllable. PA1 6. U.S. Pat. No. 4,996,462 by Krummel discloses electronic ballasts for fluorescent lamps. This design is strictly for the purpose of lowering the voltage requirement on a capacitor used in an electronic ballast. PA1 7. U.S. Pat. No. 5,004,947 by Nilssen discloses an electronic ballast with a power circuit having magnetic transformers, parallel lamps and a FET circuit. This design varies the lamp current by means of a permanent magnet changing the flux density of the saturable transformer. Movement of the magnet around the transformer can change lamp current. He limits lamp current by a variable transformer that controls frequency which in turn controls current. PA1 8. U.S. Pat. No. 5,004,959 by Nilssen discloses a fluorescent lamp ballast with adjustable lamp current. He limits current with a variable transformer that controls frequency which in turn controls current.
4. Features of the Non-Flickering Electronic Ballast
5. Reference and Prior Art Statement
The inventors have also researched the literature and discuss the following patents which may be construed as having somewhat similar function: